1. Field of the Invention
The present invention relates to an internal combustion engine for a vehicle, and more particularly to an internal combustion engine that includes at least a first fuel injection mechanism (in-cylinder injector) for injecting a fuel into a cylinder and further includes a second fuel injection mechanism (intake manifold injector) for injecting a fuel into an intake manifold or an intake port.
2. Description of the Background Art
An internal combustion engine provided with an intake manifold injector for injecting a fuel into an intake manifold and an in-cylinder injector for injecting a fuel into a combustion chamber, in which fuel injection from the intake manifold injector is stopped when load of the engine is lower than preset load and fuel injection from the intake manifold injector is allowed when load of the engine is higher than the preset load, is known.
An in-cylinder injection type engine aiming at improvement in combustion efficiency and purification of exhaust gas by making smaller particles of fuel injected into the cylinder represents one example of techniques related to such an in-cylinder injector in an internal combustion engine. For example, Japanese Patent Laying-Open No. 2003-254199 discloses an in-cylinder fuel injection type internal combustion engine that achieves ensured improvement in fuel efficiency by permitting setting of a high compression ratio even when an average air-fuel ratio of the whole air-fuel mixture in a cylinder bore is high and when the air-fuel mixture is lean on the average such as in a low load state of the internal combustion engine. In the in-cylinder fuel injection type internal combustion engine, an intake manifold is formed on one side of a cylinder head, whereas an exhaust manifold is formed on the other side thereof when the cylinder in which an axial center of the cylinder bore is aligned with a vertical line is viewed from a side. The in-cylinder fuel injection type internal combustion engine includes a fuel injection valve capable of injecting the fuel in an obliquely downward direction from an end side on one side of the cylinder head into the cylinder bore and a spark plug of which discharge portion is exposed within the cylinder bore substantially on the axial center of the cylinder bore. In the in-cylinder fuel injection type internal combustion engine, when the cylinder is viewed two-dimensionally, the fuel injected from the fuel injection valve is in an inverted V-shape with the discharge portion lying between two prongs, and the fuel is injected from the fuel injection valve in an intake stroke.
According to the in-cylinder fuel injection type internal combustion engine, the fuel injected from the fuel injection valve is in an inverted V-shape with the discharge portion lying between the two prongs. In addition, in the intake stroke of the internal combustion engine, the piston is lowered from the top dead center. This direction of lowering is the same as the direction of injection of the fuel from the fuel injection valve. Therefore, the fuel injected from the fuel injection valve travels along each outer side of the discharge portion. Here, furious collision of the fuel with an upper surface of the piston is prevented, and the fuel smoothly travels in a direction of injection. When forward ends of respective two-pronged injected fuel on the left and right reach an inner circumferential surface of the cylinder bore and the upper surface of the piston, the fuels are guided by these surfaces so that some part of the fuels comes closer to each other in a circumferential direction of the cylinder bore, while other part thereof moves away from each other in the circumferential direction of the cylinder bore. Then, in the intake stroke and the following compression stroke, most of the fuel injected into the cylinder bore is concentrated in an area in the vicinity of the inner circumferential surface of the cylinder bore substantially uniformly in the circumferential direction. Namely, when the cylinder is viewed two-dimensionally, a stratified, ring-shaped rich air-fuel mixture substantially around the axial center of the cylinder bore and a stratified, lean air-fuel mixture surrounded by the stratified rich air-fuel mixture and located in the vicinity of the discharge portion are formed in the cylinder bore.
In the in-cylinder fuel injection type internal combustion engine described above, a desired fuel spray form is maintained by supplying the fuel at a high pressure from the in-cylinder injector to maintain traveling force of the fuel with that pressure. Depending on a load state of the internal combustion engine, however, there is a fuel spray form more preferable than the fuel spray form above. Japanese Patent Laying-Open No. 2003-254199 does not disclose such a point of view.